Measurement of Λ Polarization in the π− p → K 0Λ Reaction at $p_{\pi ^{-}}=1.33$ GeV/ c toward a New Λ p Scattering Experiment

The structure of matter, i.e. the binding of nucleons to nuclei and the formation of quarks to nucleons or other hadrons, is governed by the strong interaction. The underlying Gauge theory, Quantum Chromodynamics (QCD), is well established and has a characteristic property: the coupling constant is decreasing as a function of the momentum transfer (energy). In high-energy reactions, quarks and gluons behave as free particles and the coupling constant is small. This regime of QCD, where quarks and gluons interact only weakly, is called asymptotic freedom and pertubative calculations can be used to predict interactions. However, at small energies, the quarks interact strongly and virtual gluons can produce gluon-gluon pairs and confine quarks in colorless hadrons. Due to the large coupling constant, pertubative calculations of QCD are unreliable at low energies and cannot explain the confinement. In this low-energy range, only phenomenological models such as quark models or numerical calculations (lattice QCD) can be used to solve QCD. To verify QCD models at low energies, the excitation spectrum of the nucleon is of particular interest. Comparison of the model predictions and the experimentally observed states have shown a large discrepancy in number and ordering of the levels. Many more states are predicted than have been experimentally observed, which is known as the problem of missing resonances. This mismatch may either originate from the effective degrees of freedom of the models or from experimental bias. In the beginning of hadron spectroscopy, most results have been obtained from pion-nucleon scattering experiments. However, since the intermediate nucleon resonance depends on the production mechanism, only resonances that couple to π N have been observed. In the last decades, these results have been supplemented with data on unpolarised cross sections obtained from meson photoproduction at various acceleration facilities. These results could clarify the situation to some extent. Nevertheless, the problem of missing resonances persists, which is mainly caused by the fact that many resonances are broad and overlapping. Thus, current experiments focus on the measurement of single and double polarisation observables, which may improve the situation since observables are sensitive to interference terms and thus can enhance weak contributions from resonances. In this work, η photoproduction from quasi-free protons and neutrons has been studied. Photoproduction of η mesons is of particular interest since former results of different collaborations have shown an unusual narrow structure in the cross section on the neutron, which is not visible on the proton. Various theoretical models exist that try to explain this effect, but no conclusive solution has been found yet. Thus, to get a final interpretation of this effect, unpolarised cross sections, the double polarisation observable E and the helicity dependent cross sections σ1/2 and σ3/2 have been extracted in this work. Unpolarised total and differential cross sections have been determined for protons and neutrons bound in light nuclei, i.e. deuterium and 3He. Data have been measured with the CBELSA/TAPS experiment at the Electron Stretcher Accelerator (ELSA) in Bonn (deuterium, December 2008) and with the A2 experiment at the Mainzer Microtron (MAMI) in Mainz (3He, November 2008). Both setups used energy-tagged photon beams to produce η mesons from cryogenic liquid targets. The target was surrounded by an almost 4π covering detector setup. At CBELSA/TAPS the combined setup of Crystal Barrel (CBB) and MiniTAPS was used, at A2 the main detectors were Crystal Ball (CB), TAPS. Furthermore, experiments aiming at the extraction of the double polarisation observable E, have been run at both acceleration facilities. A circularly polarised photon beam and a longitudinally polarised deuterated butanol (dButanol) target have been used. The results obtained in this work give input to new partial wave analysis and help to straighten out the situation of η photoproduction from the neutron.

We report on vibrational coherence dynamics in excited and ground electronic states of all-trans retinal protonated Schiff-bases (RPSB), investigated by time-resolved Degenerate Four-Wave-Mixing (DFWM). The results show that wave packet dynamics in the excited state of RPSB consist of only low-frequency (<800 cm(-1)) modes. Such low-frequency wave packet motion is observed over a broad range of detection wavelengths ranging from excited state absorption (∼500 nm) to stimulated emission (>600 nm). Our results indicate that low-frequency coherences in the excited state are not activated directly by laser excitation but rather by internal vibrational energy redistribution. This is supported by the observation that similar coherence dynamics are not observed in the electronic ground state. Challenging previous experimental results, we show that the formation of low-frequency coherence dynamics in RPSB does not require significant excess vibrational energy deposition in the excited state vibrational manifolds. Concerning ground state wave packet dynamics, we observe a set of high-frequency (>800 cm(-1)) modes, reflecting mainly single and double bond stretching motion in the retinal polyene-chain. Dephasing of these high-frequency coherences is mode-dependent and partially differs from analogous vibrational dephasing of the all-trans retinal chromophore in a protein environment (bacteriorhodopsin).

Excited state electronic structure and dynamics in diblock π-conjugated oligomers

A multidimensional least-squares analysis of transient Raman data acquired during a single laser pulse is used to resolve the spectra of excited-state species from the spectra of ground-state and solvent species. The kinetics of optical excitation produce a higher order dependence on laser intensity for Raman spectra of the excited states. In the absence of saturation of the ground- or excited-state populations, the excited-state spectrum increases quadratically with laser intensity. In cases where excited states cause significant absorption of the excitation source, the scattering from a solvent band may be used as an in situ intensity reference to correct the measured laser energies for the analysis. Saturation of excited-state populations causes deviations from the sample quadratic dependence of Raman intensities on laser energy. Least-squares regression analysis with a model describing the saturation kinetics allows the spectrum of the excited state to be resolved. The method is introduced in this work and applied to the detection of the Raman spectrum of benzophenone excited triplet states. A kinetic model describing the loss of triplet states through dissociation of upper triplet states of benzophenone produced during the laser pulse explains the results observed with 316 nm excitation

We report measurements of π+ and π0 meson photoproduction from longitudinally spin-polarised protons by an energy tagged (0.73-2.3 GeV) and linearly polarised photon beam. A close to complete solid angle coverage for the reaction products was provided by the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The double-polarisation observable G is extracted from Maximum Likelihood fits to the data, enabling the first accurate determination for the reaction γ→p→→π+n, while also significantly extending the kinematic coverage for γ→p→→π0p. This large data set provides an important constraint on the properties and spectrum of excited nucleon states decaying to Nπ in the mass range from 1.4 to 2.2 GeV, as well as for background (non-resonant) photoproduction processes. The considerable improvement achieved in the description of the observable G within the SAID and Bonn-Gatchina approaches after implementation of our data, illustrates that the partial-wave analyses now significantly extend the knowledge on Nπ photoproduction amplitudes at W>1.8 GeV. A partial-wave analysis using the new high-precision data set has a large impact on the extracted properties of high-spin nucleon excited states.

Measurement of polarization observables in π⁰ and π⁰π⁰ photoproduction from protons and neutrons at MAMI and ELSA

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTExcited state spectra and dynamics of phenyl-substituted butadienesStacie E. Wallace-Williams, Benjamin J. Schwartz, Soeren Moeller, Robert A. Goldbeck, W. Atom Yee, M. Ashraf El-Bayoumi, and David S. KligerCite this: J. Phys. Chem. 1994, 98, 1, 60–67Publication Date (Print):January 1, 1994Publication History Published online1 May 2002Published inissue 1 January 1994https://doi.org/10.1021/j100052a011RIGHTS & PERMISSIONSArticle Views342Altmetric-Citations21LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1002 KB) Get e-Alerts

Within the framework of AdS/CFT duality, excited states of the conformal field living at the global AdS boundary of a four-dimensional spacetime Einstein gravity are investigated analytically in the probe limit where the field equations are linearized. At asymptotically large values, the threshold chemical potential for the appearance of excited condensate states are discrete, equal spacing, with the gap approaches zero logarithmically in the limit T → 0. Remarkably, numerical results show that, this behavior applies even for states as low as for the first or the second excited state of the condensate. This is especially significant on the liquid side of the black hole van der Waals-like phase transition (small or zero topological charge) where there seems to be no gap between the ground state and the first excited state at zero temperature. We postulate that, at the exact limit T = 0 where the gap is zero, the spectrum of threshold chemical potentials becomes continuous, all excited states of the condensate are activated above a finite chemical potential, suggesting a new quantum phase transition as a function of the chemical potential. Previous studies have largely missed this continuous spectrum of excited states in the T → 0 limit. This fact should be taken into account carefully in AdS/CFT duality studies.

The study of baryon resonances provides a deeper understanding of the strong interaction because the dynamics and relevant degrees of freedom hidden within them are re ected by the properties of the excited states of baryons. Higher-lying excited states at and above 1.7 GeV/c2 are generally predicted to have strong couplings to final states involving a heavier meson, e. g. one of the vector mesons, ρ, ω φ, as compared to a lighter pseudoscalar meson, e. g. π and η. Decays to the ππΝ final states via πΔ also become more important through the population of intermediate resonances. We observe that nature invests in mass rather than momentum. The excited states of the nucleon are usually found as broadly overlapping resonances which may decay into a multitude of final states involving mesons and baryons. Polarization observables make it possible to isolate single resonance contributions from other interference terms. The CLAS g9 (FROST) experiment, as part of the N* spectroscopy program at Jefferson Laboratory, accumulated photoproduction data using circularly- & linearly-polarized photons incident on a transversely-polarized butanol target (g9b experiment) in the photon energy range 0:3-2:4 GeV & 0:7-2:1 GeV, respectively. In this work, the analysis of reactions and polarization observables which involve two charged pions, either in the fully exclusive reaction γρ -> ρπ+π- or in the semi-exclusive reaction with a missing neutral pion, γρ -> ρπ+π-(π0) will be presented. For the reaction ρπ+π-, eight polarization observables (Is, Ic, Px, Py, $P^s_{x;y}$, $P^c_{x; y}$) have been extracted. The high statistics data rendered it possible to extract these observables in three dimensions. All of them are first-time measurements. The fairly good agreement of Is, Ic obtained from this analysis with the experimental results from a previous CLAS experiment provides support for the first-time measurements. For the reaction γρ -> ρω -> ρπ+π(π0, five polarization observables (T, Σ, F, H, P) have been extracted, four of which are first-time measurements at all energies. This analysis thus represents a comprehensive program on vector-meson photoproduction: The ω is observed and studied directly from the data and the polarization observables for the (broad) ω can be extracted from the double-pion reaction in a partial-wave analysis. The 13 polarization observables extracted in this analysis substantially augment the world database of polarization observables for these reactions and are expected to play a crucial role in identifying the contributing baryon resonances.

Structures and dynamics of the lowest excited triplet states and cation radicals of phenothiazine and 2-chlorophenothiazine: transient resonance Raman and absorption study

Blue light absorbing flavoproteins play important roles in a variety of photobiological processes. Consequently, there have been numerous investigations of their excited state structure and dynamics, in particular by time-resolved vibrational spectroscopy. The isoalloxazine chromophore of the flavoprotein cofactors has been studied in detail by time-resolved Raman, lending it a benchmark status for mode assignments in excited electronic states of large molecules. However, detailed comparisons of calculated and measured spectra have proven challenging, as there are many more modes calculated than are observed, and the role of resonance enhancement is difficult to characterize in excited electronic states. Here we employ a recently developed approach due to Elles and co-workers ( J. Phys. Chem. A 2018, 122, 8308-8319) for the calculation of resonance-enhanced Raman spectra of excited states and apply it to the lowest singlet and triplet excited states of the isoalloxazine chromophore. There is generally good agreement between calculated and observed enhancements, which allows assignment of vibrational bands of the flavoprotein cofactors to be refined. However, some prominently enhanced bands are found to be absent from the calculations, suggesting the need for further development of the theory.

Transient absorption spectroscopy has been used to elucidate the nature of the S1 intermediate state populated following excitation of cob(III)alamin (Cbl(III)) compounds. This state is sensitive both to axial ligation and to solvent polarity. The excited-state lifetime as a function of temperature and solvent environment is used to separate the dynamic and electrostatic influence of the solvent. Two distinct types of excited states are identified, both assigned to pi3d configurations. The spectra of both types of excited states are characterized by a red absorption band (ca. 600 nm) assigned to Co 3d --> 3d or Co 3d --> corrin pi* transitions and by visible absorption bands similar to the corrin pi-->pi* transitions observed for ground state Cbl(III) compounds. The excited state observed following excitation of nonalkyl Cbl(III) compounds has an excited-state spectrum characteristic of Cbl(III) molecules with a weakened bond to the axial ligand (Type I). A similar excited-state spectrum is observed for adenosylcobalamin (AdoCbl) in water and ethylene glycol. The excited-state spectrum of methyl, ethyl, and n-propylcobalamin is characteristic of a Cbl(III) species with a sigma-donating alkyl anion ligand (Type II). This Type II excited-state spectrum is also observed for AdoCbl bound to glutamate mutase. The results are discussed in the context of theoretical calculations of Cbl(III) species reported in the literature and highlight the need for additional calculations exploring the influence of the alkyl ligand on the electronic structure of cobalamins.

Stationary spectra offer information on the interplay between the structures and the nature of electronic excitations reflecting bonding properties, as shown by comparing Sin with Agn (n=4-6) clusters. In order to study the dynamical properties, simulations and analysis of femtosecond (fs) time-resolved pump–probe or pump–dump signals have been carried out, which allows us to determine the timescales and the nature of configurational changes versus internal vibrational relaxation (IVR) in electronic ground or excited states. For this purpose we have developed a multi-state ab initio molecular dynamics (involving ground as well as adiabatic or non-adiabatic excited electronic states) on the timescale of the nuclear motion, using the time evolution of a thermal ensemble in the Wigner representation. The combination of ab initio quantum-chemical methods used for the molecular dynamics ‘on the fly’ and the Wigner-distribution approach for the description of the motion of the nuclei also allowed the accurate determination of pump–probe and pump–dump signals under temperature-dependent initial conditions. We use this novel combination of methods to investigate the dynamics in excited states of non-stoichiometric NanFn-1 clusters with a single excess electron. The timescales of the structural relaxation in excited states versus intramolecular vibrational relaxation processes have been determined, as illustrated for the example of Na4F3. This is the first study of the system with 15 degrees of freedom for which the dynamics in the excited states has been carried out without the precalculation of the energy surfaces.

Excited state dynamics of 5-azacytosine (5-AC), 2,4-diamino-1,3,5-triazine (2,4-DT), and 2-amino-1,3,5-triazine (2-AT) were comprehensively investigated by steady state absorption, fluorescence, and femtosecond transient absorption measurements. Time-dependent density functional theory (TDDFT) calculations were performed to help assign the absorption bands and understand the excited state decay mechanisms. The experimental results of excited singlet state dynamics for 5-AC, 2,4-DT, and 2-AT with femtosecond time resolution were reported for the first time. Two distinct decay pathways, with ∼1 ps and tens of picosecond lifetimes, were observed in 5-AC. Only one decay pathway with 17 ps lifetime was observed in 2,4-DT while an emissive state was found in 2-AT. TDDFT calculations suggest that 5-AC has a dark nπ* (S1) state below the first allowed ππ* (S2) state, which leads to the ultrafast decay of the ππ* state. In 2,4-DT, there is no dark nπ* state below the bright ππ* (S1) state and the 17 ps lifetime is assigned to the relaxation from the ππ* (S1) state to ground state. Two dark nπ* states (S1 and S2) were found in 2-AT, which exhibits much more complex excited state dynamics compared with the other two. Photoluminescence in 2-AT has been confirmed to be fluorescence emission from its bright ππ* (S3) state. Our results strongly suggest that electronic structures are very sensitive to the substitution on the triazine ring and that the photophysical properties of nucleic acid analogues depend highly on their molecular structures.

In this chapter we will review the processes at play in the excited states of protonated molecules. More specifically, the discussion will be restricted to aromatic molecules, on which the majority of studies have been focused. Rather than going into details of each system, we will highlight the trends and general mechanisms that are involved in the excited state energy and dynamics. The interested reader can find detailed descriptions of each molecule in the individual publications. It appears that the excited state dynamics of many aromatic ions can be understood upon the application of rather elementary and universal concepts, the simplest of which being that the excited electron is attracted to the proton, triggering a large part of the excited state dynamics.